Concrete columns are used in buildings, bridges and other structures to support axial compression and resist flexural and shear stresses. They are often reinforced with reinforcement consisting of longitudinal and transverse steel. The longitudinal reinforcement contributes to axial and flexural resistance. The transverse reinforcement contributes to improving shear (diagonal tension) capacity, preventing or delaying buckling of longitudinal reinforcement in compression, and confining concrete to improve strength and deformability of concrete. While the amount of longitudinal reinforcement affects flexural and axial strength, it does not play a significant role on column deformability. However, the transverse reinforcement plays a vital role on column shear strength and deformability. Columns are often required to be designed with sufficient transverse reinforcement, in the form of ties, hoops, overlapping hoops and crossties for excess shear capacity to prevent premature shear failure, which is regarded as a brittle form of failure. Hence, in properly designed concrete columns, brittle shear failure never precedes ductile flexural failure.
The same transverse reinforcement also improves flexural performance if placed with sufficiently small spacing. Closely spaced transverse reinforcement provides a reinforcement cage which confines the compression concrete. Concrete in compression develops a tendency to expand laterally due to the Poisson's effect. Lateral expansion generates transverse tensile strains and longitudinal splitting cracks which eventually result in failure. The presence of closely spaced transverse reinforcement controls the development of splitting cracks and delays the failure of concrete. Lateral expansion of concrete is counteracted by passive confinement pressure exerted by reinforcement. The resulting confinement action enhances both the strength and deformability of concrete. These improvements directly translate into flexural strength enhancement, as well as a very significant increase in inelastic deformability.
Performance of buildings and bridges during recent earthquakes indicated serious design deficiencies, especially when stresses exceed elastic limits of materials. For example, the majority of bridge failures in the 1994 Northridge Earthquake were attributed to lack of shear and/or confinement reinforcement in columns. Similarly, a large number of building failures during past earthquakes have been attributed to poor column behavior, especially due to lack of shear/confinement reinforcement. A large number of bridges were found to have seismic deficiencies in the State of California alone. These structures need to be retrofitted for improved strength and ductility.
Columns of multistory buildings are often critical at the first story level, where they may be subjected to plastic hinging due to excessive flexural stress reversals, or shear distress caused by high seismic shear forces. These columns are often fixed to the foundation, and are built monolithically with the structure. Hence, they often deform in double curvature, developing high flexural stresses at the ends, near the supports, where they are restrained against bending. These end regions may become critical for flexure. High flexural tensile stresses may develop, causing the column longitudinal reinforcement yield, initiating ductile response until compressive stresses in concrete result in the crushing of the concrete. Concrete crushing is a brittle form of failure, leading to sudden and immediate loss of strength. One viable approach to prevent the brittle failure of concrete in compression is to provide lateral confinement. Confined concrete is laterally restrained against possible expansion. Axially compressed concrete can not crush unless it expands laterally due to the Poisson effect and develops vertical tensile cracks. The lateral pressure provided by confinement overcomes the tendency to expand, improving strength and ductility of concrete. In new construction the building code requirements for internally placed transverse confinement reinforcement results in sufficient lateral confinement to improve deformability of columns. In existing columns, however, built prior to the development of current code provisions, lack of properly designed transverse reinforcement results in brittle failures. Hence these columns fail due to compression crushing of concrete unless retrofitted externally to provide the required confinement.
Similar critical regions may develop in bridge columns. These columns are built to be fixed against flexural rotation at their footings. Hence, the column end near the footing may be critical against flexure and hence compression crushing. Certain bridge columns are monolithically built with the bridge deck. These columns may also have a critical region near the deck. However, bridge columns may also have a hinge support at their ends near the deck. The latter category of columns are not subjected to significant flexure near the deck, and hence are not critical at this location.
Confined concrete also provides proper anchorage to reinforcement. Therefore, lap splice regions of longitudinal reinforcement are often required to be confined, if the bars are at or near the potential hinging regions. Hence, confining concrete also results in beneficial effects in lap splice regions.
Both building and bridge columns may attract significant shear forces if they are short. Short and stubby columns may be critical in shear, developing diagonal tension and compression failures along their heights. Diagonal tension failure in a concrete column occurs when transverse column steel is not adequate. In such a case, the column fails prematurely, prior to developing its flexural capacity. While flexural yielding and associated flexural hinging may lead to ductile response, especially if the column is well confined, diagonal tension failure results in a sudden and brittle failure. Therefore, these columns must be retrofitted externally to prevent brittle shear failure. Although rare, some shear-dominant columns may experience diagonal compression crushing of concrete if diagonal shear failure is prevented by excessive transverse reinforcement. Concrete confinement helps in this case, improving the behavior of concrete against diagonal compression.
It is clear from the above discussion that the transverse reinforcement plays a significant role on inelastic deformability of concrete columns. While properly designed transverse reinforcement is required by building codes in all new columns, its function can be fulfilled by external prestressing in old and existing columns which may not possess adequate transverse reinforcement. Retrofitting through external prestressing has the added advantage of providing actively applied lateral pressure. Active lateral pressure delays the formation of diagonal shear cracks in columns, and limits widths of such cracks, improving aggregate interlock and consequently increasing concrete contribution to shear resistance. The active pressure also increases lateral confinement and enhances the mechanism of concrete confinement, while also restraining longitudinal reinforcement against buckling.
The most commonly used prior art for column retrofitting is steel jacketing. Steel jacketing involves covering the column surface by steel plates, welding the plates to form a sleeve, and filling the gap between the steel and concrete by pressure injected grout. The steel jacket overcomes diagonal tensile and compressive stresses generated by shear, while also restraining concrete against lateral expansion, thereby confining the column for improved deformability. In circular columns, passive confinement pressure is developed from hoop tension in the steel jacket as the concrete expands laterally. However, the same mechanism cannot be utilized in square and rectangular columns, unless the column is first re-shaped to have an elliptical or circular shape before a steel jacket is put in place. The steel jacketing can be quite costly because of the large amounts of steel used and each steel jacket has to be custom made especially for non-circular columns. However, because of lack of availability of a more practical and economical technique, steel jacketing forms the majority of recent applications for column retrofitting.
Jacketing concrete columns can also be done by providing a reinforced concrete sleeve around existing columns. This technique requires placement of reinforcement cage around the existing column which may be quite cumbersome especially because of the substantial amount of closely spaced transverse reinforcement that has to be placed around the column. Another complication is to provide the formwork and place concrete in the sleeve. The mechanism of confinement and shear force resistance remains the same as that for steel jacketing.
Another retrofitting technique, that is being researched and developed for concrete columns, is fiber wrap, involving fiber reinforced polymer (FRP) materials. This technique involves covering the surface of concrete column by an FRP wrap, which provides passive confinement pressure as the concrete expands laterally under compression. While this technique was proven to be effective for concrete confinement, its use against diagonal tension caused by shear is still questionable. Furthermore, the high cost of material, the emission of toxic odors that can harm individuals in indoor applications and the lack of experience with long term durability of the material appear to be disadvantages that currently prevent widespread use of this technology. Although the application of FRP in circular columns shows promising results, in the case of rectilinear or polygonal columns, this technique has some drawbacks such as lack of concrete confinement and brittle failures at sharp corners of the columns. The above prior art techniques are discussed in the U.S. Pat. No. 5,680,739 which issued to Cercone et al on Oct. 28, 1997.
From the foregoing discussion, it is concluded that an economically viable, structurally effective and durable, and practically superior retrofitting technique is needed in the construction industry for concrete columns. The need to upgrade concrete columns remains a challenge to structural engineers, especially in seismically active regions.